What is the cheapest way to get from London to Paris, or paris to amsterdam?

I am travelling in August/September next year to london, and will be going to paris from there, then Amsterdam then back to london. Can anyone please tell me the cheapest way? Funds are low and i'm saving as much as i can, so i would rather spend the money i will have on fun stuff then on the transportation. I know that the eurostar/rail is the fastest, but i'm wondering what the cheapest is. Also, is it cheaper to book all at once or to do one way tickets all the way around? Thanks!